Device for correcting third-order spherical aberration in a lens, especially the objective lens of an electronic microscope

ABSTRACT

A device for correcting third-order spherical aberration in the objective lens of an electron microscope, including an objective lens and a correction device which is formed by two hexapoles and a round-lens doublet arranged therebetween having two round lenses with the same focal length, whereby a single round lens ( 3 ) is arranged between the objective lens ( 2 ) and the correction device ( 1 ) in such a way that a parallel optical path hits the correction device ( 1 ) and the coma-free plane ( 6 ) of the objective lens is represented on the plane of the first hexapole ( 8 ) of the correction device ( 1 ) or two round lenses with different focal lengths are arranged between the objective lens and the correction device, whereby the distance between the round lens ( 14 ) close to the objective and the coma-free plane ( 16 ) of the objective and the distance between the round lens ( 15 ) close to the correction device and the coma-free plane ( 17 ) of the correction device is the same is terms of focal length and the distance between both round lenses ( 14, 15 ) is equal to the sum of their focal lengths.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The invention relates to a device for correcting third-order sphericalaberration in a lens, especially the objective lens of an electronmicroscope, comprising an objective lens and in the direction of theoptical path a downstream correction device, which is formed by a firstand a second hexapole and a round-lens doublet arranged therebetweencomprising two round lenses with the same focal length, whereby thedistance between the two lenses is twice their focal length, each beinglocated at the focal point distance from the centre plane of therespective adjacent hexapole.

2. Description of the Prior Art

The term spherical aberration covers all those optical image defectswhich in the Gaussian dioptrics are determined by the elementary path,which originate from the optical axis in the object plane to be mapped.In high resolution electron optical systems the performance, i.e. theresolving capacity, is limited by the spherical aberration. It istherefore a principle concern in high resolution electron microscopicoptics to eliminate this spherical aberration. In the case of round-lenssystems with a straight optical axis, the defect is the third-orderspherical aberration. As a correction device, the journal ‘NuclearInstruments and Methods Vol. 187, 1981, page 187ff., has alreadyproposed a suitable solution, especially for scanning transmissionelectron microscopes. Basically, the design comprises two hexapoles,between which there is a located a round-lens system comprising tworound lenses with the same focal length, whereby the distance betweenthe lenses is twice their focal length. The two hexapoles are located atthe focal point distance from the respectively adjacent round lens sothat the centre plane of the first hexapole is imaged on the centreplane of the second hexapole.

European Patent Application No. 0,451,370 proposes a concreteapplication of this correction device to eliminate the third-orderspherical aberration for a round lens serving as an objective, wherebybetween the correction device described above and the objective lens afurther round-lens doublet is situated, which in configuration andarrangement to the entrance side hexapole of the corrector matches theround-lens doublet of the corrector.

It is to be seen as a considerable disadvantage that setting thespherical aberration of the correction device described in theaforementioned printed publication requires appreciable efforts forsetting and adjustment. Moreover, the described solution, an achievedoptimised corrector configuration can only be used in conjunction withone objective (for which it was optimised).

Adaptation to other objectives (with a different focal length andspherical aberration) is difficult or even impossible and requireschanges to be made in the corrector itself.

SUMMARY OF THE INVENTION

On this basis, it is the object of the invention to dispose thecorrection device described above, in order to eliminate the third-orderspherical aberration, in the optical path behind an objective and toenable first simple adjustment to different objectives and second asimple fine adjustment of the spherical aberration.

In order to solve the inventive tasks two solutions are proposed whichcan be implemented independently of each other.

The first solution described disposes a round lens between thecorrection device described above and the objective lens. In respect ofits strength and position the round-lens is to be configured so that itfulfils two conditions: first it must direct the optical pathoriginating from the objective lens as a parallel optical path onto theentrance of the correction device known per se; therein lies aprecondition for the fundamental path originating from the optical axisat the object point. In addition, the round lens has the task of imagingthe coma-free plane of the objective lens onto the coma-free planes ofthe hexapole of the correction device. This yields a precondition forthe extra-axial fundamental path, which has a nodal point on thecoma-free planes. It follows furthermore from these conditions that theaxial fundamental path intersects the optical axis before the round lensso that at a distance close to the front of the round lens a first imageis produced. As a delimitation to European Patent Application No.0,451,370 attention is drawn to the fact that in contrast thereto thefirst image is positioned in infinity and the use of a further roundlens is essential.

The construction of the state of complete correction requires thefollowing steps:

To that end, a correction device is presumed which is permanently andoptimally adjusted. Thereafter, the inventive round lens positionedbetween the objective lens and the corrector is moved along the opticalaxis to optimise the enlargement and to adjust the path height of theaxial fundamental path and its lens strength is adjusted. As a finalstep, the objective lens is adjusted by altering its strength. Therebythe strength of the alteration of the objective lens is comparativelysmall and for typical objectives is less than 10%. The adaptation andadjustment of the condition of full correction is thus primarilyachieved by arranging and setting the round lens in the specified way.Adjusting the corrective condition does not produce appreciabledisadvantages in respect of the coefficients of the image defects. Thedecisive advantage is to be seen therein that not the corrective deviceis to be adjusted in accordance with the spherical aberration and thefocal length of the objective lenses and the desired residual sphericalaberration, which would bring considerable and far-reaching adjustmentproblems in respect of the number of elements of the correction deviceto be changed and in respect of the latter's stability. The adjustmentproposed by the invention with the aid of the round lens is in contrastthereto implemented simply and unproblematically. In addition to thesubstantial simplification of adjustment, in respect of thestate-of-the-art there is a substantial simplification of theconfiguration of the apparatus because henceforth only a single roundlens is used. This means, for example, that for the spatialaccommodation of this round lens, which must be implemented in theobjective pole shoe and is thus limited, particularly in radial respectterms sufficient space is available. Moreover, the cooling performancerequired is substantially reduced.

The aforementioned corrective arrangement has the object of fullyeliminating the third-order spherical aberration. To complete thepicture, for clarification it is mentioned that in operation, togenerate a phase contrast, the spherical aberration is not reduced tozero, but to a value which is more or less 1% to 5% of the originalvalue. This setting of the minor third-order image defect is obtained byadjusting the strength of the round lens and not that of the correctoras is the case of the known solution.

The inventive corrective arrangement is also highly suitable for thispurpose.

A further fully independent solution of the task is characterisedtherein that the focal lengths of the round lenses of the doubletbetween the objective and the corrector are different, the distancebetween the round lens close to the objective and the coma-free plane ofthe objective and the distance between the round lens close to thecorrection device and the coma-free plane of the correction device isthe same in terms of focal length and the distance between both roundlenses is equal to the sum of their focal lengths.

In terms of the number of elements used, this solution is identical tothat specified by European Patent Application No. 0,451,370 withdecisive differences in the spatial arrangement of the two round lensesbetween the objective lens and the correction device and their focallengths. Here is expressly required, and deviating from the solutionproposed there, that the two round lenses have different focal lengthsand the spatial arrangement is implemented in such as way that thedistance between the round lens close to the objective and the coma-freeplane of the objective and the distance between the second round lensclose to the correction device and the coma-free plane of the correctiondevice and the coma-free plane of the entrance side hexapole is the samein terms of focal length. In addition, the distance between both roundlenses is equal to the sum of both focal lengths. In the borderline caseof the identicalness of the two focal lengths the last requirementmerges into the requirement in accordance with European PatentApplication No. 0,451,370.

The adjustment of the correction device to a given objective lens isachieved in way similar to the described solution. Starting from anoptimised correction device, that is, one set to a fixed value, thearrangement comprising two round lenses is placed between the objectivelens and the correction device. By determining the refractive power andthe position of the two round lenses one obtains an adjustment to theobjective lens. This means that not only the correction of a sphericalaberration of a specific value is possible for an objective lens, butthe correction of the spherical aberration can also be influenced to theaffect that one and the same correction device can compensate thedifferent values of spherical aberrations of different objective lenses.Advantageously, an adjustment of the correction device to the respectivevalue of the spherical aberration of the objective lens is notessential. A design adaptation of the correction device to therespective objective lens is thus not necessary because the adjustmentis attained via the setting and the suitable positioning of the roundlenses. Here, too, adjustment of the correction device pre-set tooptimum conditions for the respective objective lens with its respectivespherical aberration value is implemented by setting and the suitablepositioning of the two round lenses disposed therebetween.

In contrast to the aforesaid alternative situation, there areconstructional disadvantages owing to the integration of two roundlenses in a specific volume, the need for a higher cooling performanceas well as a complicated configuration as is the case in the knownsolution.

In a further embodiment it is proposed to select the focal length of theround lens close to the correction device so that is less than the focallength of the round lens close to the objective. With a such anarrangement one obtains a decrease in the corrective effect of thespherical aberration.

In the opposite case of a higher focal length of the round lens close tothe correction device one obtains an increase in the correction effect.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

Further details, features and advantages can be taken from the followingdescription part in which with the aid of the drawing a typicalembodiment of the invention is given in a diagrammatic representation.It shows:

DETAILED DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS

FIG. 1: the optical path of a corrective arrangement where a singleround lens is disposed between the objective lens and the correctiondevice

FIG. 2: an arrangement where two round lenses with differing focallengths are disposed between the objective lens and the correctiondevice.

FIG. 1 shows the optical path of the two fundamental solutions of theGaussian dioptrics in the individual optical elements of the correctivearrangements, comprising a correction device (1), an objective lens (2)and a round lens (3) located therebetween. The fundamental pathoriginating from the optical axis is designated by (4) and theextra-axial fundamental path by (5). The coma-free plane (6) of theobjective lens (2) is described by the nodal point of the extra-axialpath (5). The round lens (3) is disposed so that the path (5) isrefracted in such a way that the next nodal point comes to lie on thecoma-free plane (7) of the first hexapole of the correction device (1).The axial path (4) pierces the optical axis where a first image isproduced and is directed parallel by the round lens (3) to thecorrection device. Its path height can be influenced by moving the roundlens (3) in the direction of the optical axis.

During the operation of the total system, the fine adjustment of thespherical aberration can be effected later through suitable variation ofthe strengths of the lenses (2) and (3).

The correction device itself comprises, as is known per se, two outsidelying hexapoles (8, 9) and a round-lens doublet disposed therebetween,consisting of two round lenses (10, 11) with the same focal length whosedistance apart corresponds to double the focal length. The distancebetween the round lens and its adjacent hexapole is also equal to thefocal length. This correction device known per se serves to eliminatethe third-order spherical aberration.

FIG. 2 also describes the optical path between the objective lens (12)and the correction device (13) in detail using the fundamental paths ofGaussian dioptrics, whereby the correction device (13) is identical tothe correction device (1) described above so that to avoid repetition itis not described again.

In contrast thereto, between the objective lens (12) and the correctiondevice (13), there are now located two round lenses (14, 15) ofrespectively different refractive power. The arrangement is composed inthat the first round lens (14) is disposed at a distance to thecoma-free plane (16) of the objective lens (12) which is equal to itsfocal length. The second round lens (15) is on the one hand disposed ata distance equal to the sum of the focal lengths of the two round lenses(14, 15) to the round lens (14) on the objective side and on the otherhand at the distance of its focal length to the coma-free plane (17) offirst hexapole (18) at the distance of its focal length. For a pre-setcorrection device, adjustment is effected via setting the refractivepower and defining the position of the two round lenses (14, 15) inrelation to the respective objective lens (12). While retaining thesetting of the correction device, adjustment is made to the respectivevalues of the spherical aberration of the respective objective lens (12)so that it is possible to use one and the same correction device (13)for objectives with different values for focal length and sphericalaberration.

In FIG. 2, path “A” shows the focal length of the second round lens (15)as being greater than the focal length of the first round lens (14) forincreasing the correction effect of the correction device (13), whilepath “B” shows the focal length of the first round lens (14) as beinggreater than the focal length of the second round lens (15) for reducingthe correction effect of the correction device

As a result both figures show solutions by means of which with minoradjustment work a pre-set and optimised correction device can beadjusted to the respective objective lenses, whereby with the aid ofround lenses the desired residual spherical aberration can be set.

What is claimed is:
 1. An apparatus for correcting a third-orderspherical aberration in a lens, comprising: an objective lens; acorrection device positioned in a downstream direction of a propagationof optical rays, said correction device comprising a first hexapole anda second hexapole with a round-lens doublet being arranged between saidfirst hexapole and said second hexapole, said round-lens doubletcomprising a first round lens and a second round lens of equal focallengths, with said first round lens and said second round lenspositioned with a distance between one another equal to two focallengths of said equal focal lengths, said first round lens being adistance of one said focal length from said first hexapole and saidsecond round lens being a distance of one said focal, length from saidsecond hexapole; and, an additional single round-lens positioned betweensaid objective lens and said corrective device, so that a paralleldownstream path of optical rays strikes said correction device with anaberration-free plane of said objective lens being represented on anaberration-free plane of said first hexapole of said correction device.2. The apparatus for correcting a third-order spherical aberration in alens according to claim 1, wherein said objective lens is an objectivelens of an electron microscope.
 3. An apparatus for correcting athird-order spherical aberration in a lens, comprising: an objectivelens; a correction device positioned in a downstream direction of apropagation of optical rays, said correction device comprising a firsthexapole and a second hexapole with a round-lens doublet being arrangedbetween said first hexapole and said second hexapole, said round-lensdoublet comprising a first round lens and a second round lens of equalfocal lengths, with said first round lens and said second round lenspositioned with a distance between one another equal to two focallengths of said equal focal lengths, said first round lens being adistance of one said focal length from a center plane of said firsthexapole and said second round lens being a distance of one said focallength from a center plane of said second hexapole; and, a third roundlens and a fourth round lens positioned between said objective lens andsaid correction device with said third round lens having a differentfocal length than it said fourth round lens, said third round lens beingcloser to said objective lens than said fourth round lens and saidfourth round lens being closer to said correction device than said thirdround lens, with a distance between said third round lens and anaberration-free plane of said objective lens being equal to a focallength of said third round lens, a distance between said fourth roundlens and an aberration-free plane of said correction device being equalto a focal length of said fourth round lens, and a distance between saidthird round lens and said fourth round lens being equal to a sum of saidfocal length of said third round lens plus said focal length of saidfourth round lens.
 4. The apparatus for correcting a third-orderspherical aberration in a lens according to claim 3, wherein saidobjective lens is an objective lens of an electron microscope.
 5. Theapparatus for correcting a third-order spherical aberration in a lensaccording to claim 3, wherein said focal length of said third round lensis greater than said focal length of said fourth round lens for reducinga correction effect of said correction device.
 6. The apparatus forcorrecting a third-order spherical aberration in a lens according toclaim 3, wherein said focal length of said fourth round lens is greaterthan said focal length of said third round lens for increasing acorrection effect of said correction device.